Questions tagged [digital-communications]

Related to all communication systems that use a given digital sequence as an interface between the source and the channel input.

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Why does a UART message appear on sender XBee's input instead of output?

I have a XBee3 Mesh Kit (SMT) that comes with three XBee3 SMT. I upgraded each Xbee3 SMT to the latest firmware (Zigbee-100D) in XCTU, and configured one module to be the coordinator and the other two ...
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Latency in router [closed]

Suppose 2 data packets arrive at the same time in a router and they havent been sent any data packets until that time.If we make the router table , we can choose whichever data packetb we want to have ...
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Hamming code question

Suppose we have m data bits and n parity bits. In order for the Hamming code to function: $$m+n<2^n-1$$ If we have 3 parity bits we can have up to 4 data bits. But lets say we don't have 4 data ...
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The influence of fiber length stability in coherent fiber communication

The length of optical fiber tend to change along with the environment. I wonder how do the coherent optical fiber communication (like QAM, QPSK, etc.) successfully recover the phase of the signal, ...
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Why is this 75 ohm resistor present in the Ethernet connector? (10/100Base-TX)

In this datasheet (and in other similar) pins J4, J5, J7, J8 are connected to the ground with a resistor and cap. For 10/100Base-TX pinout it seems they are not used (at least for modern of the ...
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Unintentional Beamforming

Suppose I have two cross polarized transmit antennas sending the same signal in a rather line-of-sight dominated environment to a receive antenna, is there any chance of creating unintentional ...
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Multipath destructive and constructive fading and the law of conservation of energy

Multipath in a Rayleigh fading environment causes fading, this fading could be destructive or constructive at a certain point or location in the far field, how does that align with the law of ...
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>1Mbps UART core on Lattice's ICE40UP-5k FPGA?

I need a UART core able to receive in the ~4-5Mbauds range (the transmitter is able to transmit up to 30Mbauds, over RS422 upstream of the FPGA), but Lattice's 16550 IP core only allows me to ...
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Coding and SNR gain

How does coding reduce the amount of SNR required to detect data for a certain modulation and at a certain bit error rate and does that imply that coding reduces noise power?
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How Nic process packets

As packet arrives Nic detect it on its interface like wire/air and than packet is kept in Rx FIFO the on chip memory for further processing like offloading and determining it belong to it after ...
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Costas Loop QPSK/4QAM

I can use a Costas Loop, modified for QPSK/4QAM and recover the frequency and phase successfully when using a pattern of all 1s or all 0s or a pattern that is repetitive for each I and Q data rail (in ...
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1 answer
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74HC74D initial output value after power up

I use a touch sensor (Active Low) to light up a light bulb. here I've used a 74HC74D D flip flop to turn on an off the bulb with the sensor pulse (sensor output is active low which is then converted ...
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3 answers
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Is AXI a good interface for outside communications?

Suppose if we want to get some data from some ADCs, which are placed in a separate chip, and process that data in RISC-V processor. I was looking for an interface to define/use inside the RISC-V chip ...
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2 answers
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Detect pauses in a digital signal on a scope?

I am currently using a USB-SPI converter and I would like to know if there are stutters caused by some buffer starvation or anything else, i.e. pauses in the stream of digital data. Is there a trick ...
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Isolation barrier in digital signal isolator

Could anyone comment on why a capacitor is better than an inductor according to the statement below: "In general, an isolation barrier implemented with capacitors is better for passing signals to ...
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How to convert data signal line coding from RZ to NRZ?

I have a user data stream at 10 kbps and line coded as uni-polar, 0-5 Volts, RZ (return to zero). What methods are there to convert this signal to an NRZ (non return to zero) format? The only way I ...
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How should I understand this interpretation?

Studying spatial diversity in telecommunication engineering (Fundamentals of Wireless Communication, D. Tse and P. Viswannath), I read this paragraph. $${\text {In}}\,{\|}{\textbf h}{\|}^{2}{\text {...
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How can I read an RFID tag at the back of a phone FROM THE FRONT?

I'm discovering RFID applications and I would like to be able to read an RFID card I strapped to the back of my phone (the most convenient location for me) by just putting the phone face down on the ...
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Trouble understanding length matching requirements

I am having trouble understanding where manufacturers are coming up with their length matching requirements. For example, I am looking at length matching for RGMII between a MAC and PHY. My ...
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Convolution between bit stream sampled by N number of samples per bit with a channel modeled as low pass filter

I generated a bit stream in MATLAB, from the transmitter to be convoluted by the channel I use MATLAB function "randi" between 0 & 1, so it generates random vectors of 0s & 1s and ...
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Can I reconstruct the channel impulse response from OFDM least square channel estimation?

I'm working on a successive interference cancellation (SIC) project for an OFDM system. I used pilots and least square channel estimation to estimate the channel frequency response (CFR.) It works ...
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1 answer
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Proof of Shannon capacity theorem

I was looking for a formal proof of the Shannon capacity theorem, which states the condition which is the maximum data rate possible between two wireless channels. I could not find the proof. Does it ...
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6 votes
4 answers
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Algorithm for compressing a stream of 12-bit ADC data in a Cortex M0 microcontroller

In my application, a Cortex M0 microcontroller currently samples an ADC signal at 10 kHz and sends the result via a USB serial cable to a computer. I want to now increase the sampling rate to over 100 ...
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1 vote
1 answer
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How is a GPS receiver able to calculate SNR?

I know that there is a certain relation between bit error rate and SNR for every modulation scheme. But i wonder how a GPS receiver can measure bit error rate and how it able to calculate SNR (I could ...
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How are the bits transmitted over the wire?

This should be a basic but fundamental question. Suppose I have a source and destination communicating over a wire. Source wants to send bit stream 01101 to ...
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Are there any noise cancellation algorithm used in wireless communication? if so in which layer?

I understand that there are 7 layers in the OSI model. I would like to know is there any noise cancellation algorithm used in wireless communication? If so in which layer is it used? I am looking for ...
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4 answers
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How to name the frequency of digital signal?

Let's imagine that we have some MCU in which each GPIO pin can be set high or low with a single assembler instruction (BSET/BCLR for example). If we divide one by the instruction execution time we get ...
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Question regarding bandwidth of digitally modulated signals

Assume that there are two carrier frequencies, 20 GHz (RF) and 200 THz (Optical). Both of these carrier waves are modulated by a digital signal using QPSK scheme. Consider the frequency spectrum (or ...
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Digital up conversion and digital down conversion

Description: I’m trying to use the standard sample rate of 61.44 MSPS on both sides of my digital transceiver(DUC and DDC,) so I’ll upsample my data from baseband data rate 3.84 MSPS to 61.44 MSPS ...
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Do high-speed asynchronous to synchronous UART converters exist?

I need to send synchronous serial data with control characters and 8b/10b encoding at about 5Mbaud - and I need to send that from a laptop; I do have UART ports which can keep up with these speeds, ...
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Calculate the minimum data rate required for transmission of EEG signals (0.4-64.4Hz)

I'm a little bit confused here. I assumed that the transmission is going to be PCM, so I calculated this way: I assumed number of bits (n) and sampling frequency (64.4 * 2) then I said that the ...
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CAN BUS : CAN_BUSY returned but other node recieved the previosus message

I have two can nodes in a CAN bus. node1 send messages periodically , node2 received only the first message (I can check with a CAN tool) for the first message, CAN_OK is returned for send routine. ...
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Why is Line coding called "Line coding"?

Why is line coding also called digital base band modulation named "line coding" where does it come? and how is the name it to modulation
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1 answer
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What is the difference between Antenna gain and Channel gain? [closed]

I have a fairly simple question that I've written in the title but just writing this body to meet stack's 'quality standards' What is the difference between Antenna gain and Channel gain?
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1 vote
2 answers
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Indoor SISO wireless Rician channel performance

In wireless communications and when you have a SISO system, multipath is considered to be an impairment according to theory. Suppose now that we have an indoor SISO Rician channel (a strong line of ...
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Is it advisable to make communication re attempts in modbus?

In my system, I know that the approximate response time between master-slave is 10ms, if the slave does not receive communication during those 10ms then it detects an error and the motors turn off. Is ...
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2 answers
119 views

Digital high-speed serial synchronization

How do high speed digital transceivers which do not share a common clock synchronize? Particularly ones which do not have an ADC front end? Would you just oversample the signal using a digital input, ...
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1 vote
2 answers
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Advantages of I and Q receiver over "real" signals receivers

I wanted to know better which are the main advantages of using an I and Q receiver like that: rather than a simple demodulator whose output is a real signal like that: Here some advangages are ...
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what's the necessary condition to PSK-8 system have the same error probability of a QAM-8 system?

I was solving a problem about digital modulations from my university exam that consider a QAM-8 system with adjacent signals coded by gray code, I know that the error of this system is: $$ P_{QAM-8} =...
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-1 votes
2 answers
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Analog versus digital (I2S) audio transmission over 1-2m

I need to get a microphone signal to a microprocessor about 1-2m away via cable and I need it to be digitized at some point in this chain. I'm looking for a solution that draws something on the order ...
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3 votes
5 answers
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Ways to send data with CRC validation

The data frames that are always sent using CRC are as follows: a CRC calculated for each byte (DATA) But to make the connection faster, could you send a single CRC for a whole frame of data? the ...
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1 vote
2 answers
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Testing the reliability of RS485

I have implemented an RS485 communication with CRC data validation and some algorithms to manage errors when the data received is lossy. How can I test the packet loss during RS485 communication? Is ...
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1 vote
1 answer
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Transmit OFDM EVM Measurement

At the same output power level of a power amplifier and for the same number of subcarriers would the measured EVM be different for a QPSK - OFDM signal than a 64 - QAM OFDM signal?
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What is the bandwidth formula for pulse code modulation (PCM)?

The image below shows the formula for the bandwidth of pulse code modulation (PCM.) There are two questions I want to ask: Bandwidth of the PCM signal waveform is bounded by \$ B_{PCM} = \frac{1}{2}(...
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1 vote
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How can I multiplex the outputs of an XBEE and a MAX485's?

I have a microcontroller that gets fed with 8-bit commands that either come through wires (using the RS-485 standard) or by the RX pin of an XBEE (without using actual switches) I want to multiplex ...
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Bandwidth , C/No, FEC and Link budget

While computing Eb/No do I take data rate as the input to the FEC or do I take the data rate as the output from the FEC, i.e., with the overheads. Isn't the Eb/No implicit in the overall coding gain ...
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3 answers
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Signals transmission

If I have more than one sensor that provides a digital output (1 or 0 only,) and I only have one pair of wires connected from all the sensors to a specific point (assume the sensors are at point A and ...
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1 vote
1 answer
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How to send out signal on two wires

I have two wires for a DALI communication bus. The voltage between them is 16V and I want to send out commands on the two wires from the bus. My guess would be to have a simple transistor between the ...
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3 votes
5 answers
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Handling simultaneous messages in wireless communication

I have a generic question regarding wireless communication and what techniques can be used to detect if two transmitters send the message at the exact same time. Since this is generic question and ...
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1 answer
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Calculating antenna beam footprint from frequency

I need to compute a beam footprint, but I know only the frequency of the beam. How can I compute the footptint?
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